1. Field of the Invention
The present invention relates to a direct current motor drive apparatus, and for example, to a drive apparatus of a direct current linear motor.
2. Description of the Related Art
Direct current linear motors are frequently used to move an object to be moved with high accuracy in a linear motion mechanism such as a machine tool or industrial robot. This type of linear motor is composed of a primary side and a secondary side, wherein, for example, the primary side serves as the power supply side and the stationary side, while the secondary side serves as the moving side.
The power supply side that composes the primary side is equipped with an excitation coil and a drive circuit and so forth for supplying drive current to this excitation coil. In addition, a field magnet is mounted on the moving side that composes the secondary side. The moving side, or secondary side, is moved with respect to the primary side by the thrust produced between the primary side and the secondary side based on Fleming's right-hand rule as a result of current being supplied to the above-mentioned excitation coil.
FIG. 1 shows the constitution of a drive circuit for supplying drive current to the excitation coil mentioned above. In FIG. 1, 1 is an input terminal to which a torque command signal of .+-.V.sub.T is supplied. The torque command signal of .+-.V.sub.T is supplied to absolute value circuit 2 from input terminal 1. The absolute value output of the torque command signal generated by this absolute value circuit 2 is applied to pulse width modulator 3 where it is converted into a pulse width modulation signal. The pulse width modulation signal from pulse width modulator 3 is then supplied to direction changer 4.
Direction changer 4 changes the output polarity of the pulse width modulation signal produced by pulse width modulator 3 according to a signal from polarity distinction circuit 5. The torque command signal of .+-.V.sub.T applied to input terminal 1 is then supplied to this polarity distinction circuit 5. Thus, with respect to the output signal produced by direction changer 4, the output polarity of the pulse width modulation signal is changed according to the polarity of the torque command signal applied to input terminal 1.
The pulse width modulation signal from the above-mentioned direction changer 4 is respectively supplied to the controlled input terminals of Hall effect elements 6a through 6c. The Hall effect output terminals of each Hall effect element 6a through 6c are connected to respective power amplifiers 7a through 7c. The outputs of each Hall effect element 6a through 6c are respectively amplified by these power amplifiers 7a through 7c, and then supplied to each excitation coil 8a through 8c connected to each of the output terminals of power amplifiers 7a through 7c.
Here, each of the above-mentioned Hall effect elements 6a through 6c is arranged so as to correspond to a field magnet (not shown) that composes the moving side. Thus, a pulse width modulation signal corresponding to a torque command signal is output to each power amplifier 7a through 7c only when the field magnet of the moving side approaches. Thus, output from each power amplifier 7a through 7c is supplied, in order, to each excitation coil 8a through 8c. As a result, thrust is applied to the field magnet based on Fleming's right-hand rule, and this thrust acts by moving the moving side, or secondary side, with respect to the stationary primary side with the torque corresponding to the torque command signal.
However, according to the drive apparatus of the prior art described above, the current amplified by each power amplifier 7a through 7c corresponds to pulse width intermittently. However, since the pulse width modulation signal supplied to each power amplifier 7a through 7c is produced by each Hall effect element 6a through 6c, the output of each of these Hall effect elements 6a through 6c is controlled according to the number of lines of magnetic force produced by the field magnet that composes the moving side.
The number of lines of magnetic force that act on each Hall effect element 6a through 6c is inversely proportional to the square of the distance between the field magnet and Hall effect elements. Thus, output characteristics are produced such that, as shown in FIG. 2, the output from each Hall effect element 6a through 6c rises linearly between time (I) and time (II), crest value (h) remains constant during a prescribed time t from time (II) to time (III), and then output falls linearly between time (III) and time
As a result, the operation of each power amplifier 7a through 7c is in the region of roughly complete switching during a prescribed time t from time (II) to time (III). Thus, collector loss of a power transistor that composes power amplifiers 7a through 7c does not occur to a large extent. However, in linear region (V) from time (I) to time (II) as well as linear region (VI) from time (III) to time (IV), each power amplifier 7a through 7c is not operating in the form of a complete switching operation, thus resulting in an increase in collector loss of the power transistor composing power amplifiers 7a through 7c.
Thus, this collector loss results in generation of heat by each power amplifier 7a through 7c thereby resulting in the shortcoming of reduced reliability of the drive circuit.